1. Field of the Invention
The present invention relates to a device for reducing the noise emitted by a conduit, in particular a conduit belonging to an aircraft system (air conditioning, starter-generator, auxiliary power unit).
When an airplane is in the replenishing phase (airplane on the ground and engines off), various pieces of equipment may still be running, in particular:
the air-conditioning system, which makes it possible to regulate the cabin temperature;
the starter-generator, which generates the electricity necessary for the operation of the airplane equipment—in particular air conditioning—when the engines are off;
the APU (Auxiliary Power Unit), which actuates the starter-generator.
These pieces of equipment generate noise around the airplane. This noise exits through the air intake and outlet conduits of those pieces of equipment. It is responsible for noise annoyance affecting the ground personnel (maintenance operations, etc.) and clients moving around the airplane.
2. Description of the Related Art
To reduce these annoyances, the International Civil Aviation Organization (ICAO) has defined recommendations specifying outside noise thresholds that may not be exceeded around the airplane on the ground. Additionally, pressure from airport authorities is increasing to reduce ground noise.
The techniques typically used to reduce the noise of the equipment in question are based on the use of layers of acoustically absorbing materials, positioned on the wall of the conduits or on additional partitions provided inside those conduits.
These existing technologies have several drawbacks:
a) attenuations that are sometimes insufficient: their effectiveness is directly related to the amount of absorbing material used. To significantly reduce noise, it is necessary to cover a very large portion of the walls of the conduits. This often proves problematic, since certain areas of the conduits cannot be treated due to:
the local geometry of the conduit (bends, connections between the parts of the conduit, corners, etc.);
a lack of space around the conduit to integrate a sufficient thickness of material therein.
The conduits for the equipment are also very short. The conduit surface area that may be covered is thus sometimes too limited for effective noise reduction.
(b) bulk constraints: the absorbing materials have thicknesses of several centimeters, which are added to the diameter of the conduits. Their integration involves constraints regarding the positioning of adjacent elements (fasteners, structural elements, other conduits, etc.) so as to free the necessary space.
(c) mass constraints: the large thicknesses and surface areas necessary for the absorbing materials involve significant mass penalties, which penalize the airplane. This is even more accentuated when the treated partitions are added into the conduit.
(d) thermal constraints: the flow passing through certain conduits is heated, which involves choosing absorbing materials that are compatible with high temperatures. These materials often prove heavier and less acoustically effective, which makes these technologies difficult to apply and lower performing in heated conduits.